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There are plain text versions for that as well.

Not sure it's so simple. Is LaTeX plain text? Officially it is, but by the same standard so is PostScript or PDF.

Truly the limits are blurry. Even assembly code has an ASCII representation.

Well, plain LaTeX is quite readable, so IMO, it's good enough of a compromise. It does get difficult with a lot of parentheses though.

Which example did you have in mind?

http://asciimath.org/ is the best one I've seen, by far.